3 main mechanisms:
a range of hills is turned into islands by rising sea levels
tough areas of rock are turned into islands when the surrounding softer rock erodes away
A weakspot in the earths crust forms a series of volcanoes as the plate above it moves past.
An archipelago is a group of islands more or less closely situated to each other. Causes for formation are diverse: Say... you can find them in a voulcanic activity zone as the Pacific Rim. Also you can find archipelagos wherever there was once a low sea level in a hill/mountain peak chain. Water floods in and isolates the portions of land.
Atlantis- the lost continent is supposedly to have survived as an archipelago such as the Cape Verde.